ISO

  ISO is usually stated on the camera as ISO 100, 200, 125, 160 etc. all the way up to 51000 each camera manufacturer may have variations.   It identifies the sensors sensitivity to light.  The lower the ISO number the less sensitive to light it is.  The higher the number the greater the sensitivity to light.  Also the higher the ISO the greater the chance for digital noise.  The noise can become so bad that it will make the picture ugly.  See discussion with examples in a previous post here:  http://grangerphotography.blogspot.com/2011/02/getting-that-camera.html

This leads to  the ISO balancing act.   You could use a high ISO and shoot, but then you would risk digital noise.  On the other hand you could shoot with lower ISO, and risk underexposed photographs.   The challenge then is to find the right balance.  Usually I will shoot at the lowest ISO I can to get a properly exposed photograph.  In this way I will avoid unwanted noise.  I have provided three examples.  The first photograph was shot at 100 ISO and is overexposed, in the second the ISO is 800 and is correctly exposed,  and the final photograph is overexposed.  In the final photograph the digital noise is quite noticeable if you click on it twice and look at the cabinet.

If you recall in the discussion regarding shutter speed:   http://grangerphotography.blogspot.com/2011/03/shutter-speed.html  we can control the amount of light getting to the sensor.   To correct the underexposed photograph above I could have slowed the shutter speed, and the first photograph would not have been underexposed.  In the final photograph I could have used a faster shutter speed and it would not have been overexposed.   Let's look at a couple of real examples:

There were a number of egrets and they were quite active.   I wanted to freeze action and used a shutter speed of 1/4000 of a second.  To achieve this I set my ISO to 160.

IN this photograph the swan was rather docile.  Again I wanted to freeze action, but I was not that concerned, and used shutter speed 1/250.  The ISO was 100. 

In this photograph it was just after daylight.  I set the ISO to the highest useable setting on my camera 800.  I set the shutter speed to 1/50.  I knew the shutter speed was iffy ( is that a word).  So I took a bunch of photographs hoping to catch them when they weren't moving.  I got lucky.  

This photograph was taken in a dimly lit room.  I used ISO 800 and shutter speed 1/20 of a second.  You can see the motion of his hand was not stopped by the shutter speed.   I consider it a useable photograph.

 

In this photograph ( last example) I used ISO 200 even though it was the middle of the day because I wanted a fast shutter speed.  I selected 1/3200 of a second to stop the action of the players and ball.

ON newer cameras useable ISO is getting higher and higher.  In other words there is less risk of digital noise making the selection of high ISO's less of a concern.   For my camera ISO 800 is about the top, but there are cameras now that are achieving  ISO of over 12000.  

To summarize:  In low light a slow shutter speed can be used or a higher ISO.  A slow shutter speed risks motion, and a high ISO risks digital noise.  

In bright light  a faster shutter speed and/or a lower ISO can be used, giving the photographer greater flexibility.   The photographer may choose just about any combination of shutter speed and ISO to get the right exposure.  

Next I will discuss F-stop.  It is the last way to control light in camera.